Method of polishing metal surfaces



Feb. y25, v 1947.

s. E. EATON 2,416,294

METHOD 0F POLISHING METAL SURFACES Filed Feb. 2. 1943 Patented Feb. 25, 1947 p METHOD Oli" POLISHING METAL SURFACES Samuel' E.` Eaton, Brookline, Mass., assigner to Arthur Dr Little, Inc., a corporation of Massa@ chusetts Application. February 2, 1943, Serial N0. 474,437

7 Claims. (Cl. 204-1'41) This invention relates to the discovery of an improved method oi polishing certain metals by the action of electrolysis.

It has been observed by E. S. Hedges (Chemical Society Journal, (London), r1926, pages 1533-46 and 2580-2595 that copper and silver, for example, as anodes in an electrolytic cell, upon being subiected to the action of low current densities (of the order of 60 to 145 milliamperes per square centimeter with respect to copper), present the phenomenon of forming surface coatings which rapidly alternate, which are. respectively, light and dark, and which accordingly produce a visible flashing effect. These lm 'formations are accompanied by corresponding fluctuations in potential and conversely in the current density of the cell, and the range of such fluctuations increases with increased potential. But. with a current density of the order of 145 milliamperes per square centimeter under consideration therein and a potential difference of l2 volts, such fllm formations and flashings cease and after passing through this cycle the potential of the cell rises continuously while the current density falls continuously and the film remains permanently upon the metal. This alternating film formation is attributed to the formation and separation of relatively insoluble salts of the metal such as cuprous chloride and silver cyanide, which are white, followed by the dark films which are attributed to the formation of the corresponding metallic oxides.

It is now discovered that if an object (presenting certain metallic surfaces, namely of silver, copper, or cadmium or of alloys consisting of two or more of these metals or of alloys consisting primarily of one or more of these metals, though with some zinc or nickel present therein in their composition) is subjected, in an electrolytic cell containing a soluble simple cyanide and in which such surface constitutes the anode (which is at a free space from the cathode and from other solid surfaces sufficiently to prevent the effects thereof which are found to promote periodicity or flashing) to a voltage above the polarization potential but below that at which the relatively water insoluble films would tend to form and reforinperiodically as above described, or permanentlyaccumulate, and diiusion is, per se, sufficient or further promoted by agitation of the electrolyte, and such conditions are maintained for some time, that is` until an appreciable amount or proportion of the metal is removed and dissolved in the electrolyte, the metal surface thus treated will be prevented from undergoing 2 the formation of insoluble films and be caused toy acquire a high lustrous polish.

'Ihe conditions under which such electropolishing may be effected are determined by immersing the surface to be polished, as the anode, in an electrolytic bath containing essentially a soluble simple cyanide (such as an alkali metal cyanide) and, with or without agitation, subjecting to an increasing potential (and current density) to a value greater than that at which polarization occurs, and below that at which the relatively water insoluble films would tend to form and re-form periodically or permanently accumulate.

and potential employed, and the like, under which l the determination is made. Above a certain point, further increase in the potential difference between the poles of the cell without correspond- Aing increase in current density` also leads to "permanent nlm formation, as indicated by disother hand, excessive potential or current density may cause gassing at the anode or formation of the insoluble illm.

On the other hand, agitation of the electrolyte promotes diffusion of the dissolved metal from the anode, raises the current densities to be employed at a given potential, for best results, Aand assures more `vreproducible results in effecting polishing of the metal surface.

Moreover, it is now found that whereas turning the current on and off, or repeatedly altering it, close proximity or approach of the anode to present in the electrolyte.

the cathode or to the side walls of the retaining vessel or like solid surfaces, or variations in the kind or degree of agitation of the electrolyte promote the flashing effects above noted, the gradual change or continuous control of the cur-` rent and of the agitation of the electrolyte and provision of sufficient free space in the electrolyte about the anode will effect or promote an electro` polishing action uponthe surfaceof the anode, instead of the formation ci!` such resistant dull films, either temporary or permanent.

rWhile the presence of a soluble cyanide is i clining gradient of concentration as the distance increases but very slightly therefrom. Accordnecessary 4to the procedure of electropolishing in l accordance with this invention, some of it at least must be present in the form of a free, soluble simple cyanide, with or without complex ions of the metal or metals to be`polished. Accordingly, bothV free cyanide ions and complex ions are The free alkali cyanide is the soluble cyanide which is present in excess of that required to form and maintain the complex cyanide ion of the metal which disl v solvesp The free cyanide promotes the conductivity of theelectrolyte and hence increases and maintains the current density of the cell at a-higher value at any'given potential than wouldv be produced if it were omitted. Further, increasedcondition of an inert electrolyte such as potassium ductivity may also be accomplished by the ad-V carbonate or sodium sulphate. But of these sodium sulphate is to be preferred since it is more inert than the carbonate and is more economical.

The soluble cyanide also performs the specific and important novel function, in the practical application ofthe invention for electropolishing,

cf forming both soluble and insoluble compounds with the metal of the anode surface. This is insoluble cyanldes. Consequently. a completeinsoluble film formation will not occur under such circumstances, but, on the contrary, thel irreg- 1 ularities or mountains will be removed and disy solved until the remaining surface is of uniform, dense and mirror-like or specular with respect-to the reflection of light. f

The concentration of the anode lm of electrolyte, with reference to the ions of the metal of the anode, will be greatest immediately adjacent to the.anode surface. It presents a sharply deingly, extremely minute protuberances of metal from the general surface of the anode are sufli-v cient to project through such thin film and be subjectedy to the action of `,i'llm's of electrolyte which are of less concentration with respect to silver ions, for example, but of much greater concentration with respect to free cyanide ions.

Hence, such projections will be subject to the selective dissolving action-of the cyanide ions thereon to form soluble complex (e. g., silver) cyanide ions. As a result of such action, the protuberances of metal may be dissolved progressively, in accordance with the present invention, the higher protuberances first, followed by those of less height until finally the entire metal surface is reduced to a continuous surface and reflection of light therefrom is substantially specular and uniform throughout, giving it mirror brilliancy.

As an aid to such electrolysis, dissolution and polishing off of the high points of the anode metal surface and subsequent diffusion of the dissolved utilized to advantage when the potential and current density are set to maintain a balance between the metal-dissolution rate and the rate of diffusion of the metal complex ions adjacent to the anion into the body of the electrolyte, leading, it is believed, to the formation in the 10W valleys of the metal surface of the insoluble salt and/orp oxide, and to the formation of soluble salts on the high mountains of the metal sur-` face, wherepan excess of free cyanide is more 'apt to be present because of diffusion or agitation.

It is a matter of actual observation that the film or thin layer of electrolyte solution adjacentv to the metal surface of the anode, under such conditions, presents anmarkedly different indexv of light refraction from the rest of the electrolyte solution. The presence of such a liquid film is a prerequisite for the polishing action to be induced and maintained so as to be carried to completion. The phenomenon of polishing is accompaniedby a loss of metal to the electrolyte,

and such polishing is in fact actually measured in part in its completeness and brilliancy by the time of operation and amount of such dissolution of metal from the surface being polished.

With cyanides, therefore, although they may be capable of forming insoluble salts with the metal ofthe anode, it is now found thatthey are also capable of forming soluble complex anions with the metal and going into solution under appropriate conditions of electrolysis. Hence, if diffusion of the metal cyanides into the electrolyte and of "free cyanide ions into the uid film of electrolyte adjacent to and contacting the anode is effected, at sufficient rate and concen` tration, soluble ions of the metal will be formed in this film'instead of or more rapidly than the metal ions into the body of the electrolyte (as distinguished from complete film formation by precipitation upon the anode) and diffusion of fresh cyanide ions into contact with remaining high points in the metal surface, agitation of the electrolyte is effective. But such agitation should be carefully controlled, sothat each portion of the surface will have the same degree of agitation and therefore'require and utilize the same current density. It should bedisposed in streamline relationship, as nearly as practicable, to the metal surfaces, Typically the agitation is uniformly in one direction rather than back and forth. In this way the liquid lm adjacent to the anode surface is not disrupted and yet its effective thickness may be vthus reduced and controlled. Its

concentration gradient, therefore, is believed to be increased and the metal protuberances of the surface (thus passing through a thinner layer of the protective concentrated metal ion layer of`electrolyte) are subjected to greater dissolving action, andsuch action is effective more closely to the-main body surface, that is, to the first continuous surface above which all of the protuberances, project. This surface will, therefore, ultimately constitute the finished, polished free surface of the metal when the polishing action has l been completed'by the reduction and removal of all of the visible or light diffusing or dispersing protuberances which stand above it.

In order that the current density may be thusY y set up with a relatively low potential acrossthe cell, an amount of .free soluble simple cyanide,

such as one of the alkali-metal cyanides,v in solu` y tion in excess of the cyanide ,whichI will continue` with the dissolved metal salt to form a soluble f anion therewith, as above stated, will be eifec.y tive, but an added electrolyte may be employed. In any event, however, the ffree cyanide present must be eqivalent to at least .01 mol per liter, us-f ing potassium cyanide, for example, and may be increased up to saturated solutions.

Since increased temperatures and decreased concentrations of the metal to be.polished in the electrolyte tend to increase the current density at which the critical point is reached in any given case, room temperatures or lower and relatively high concentrationsof solution are more efficient, preferably a fraction of a mol per liter.

In practicing the invention, the article to be polished is made anodic and provided with sufficient free space about it in a bath containing water soluble simple cyanide radicals and a current is passed through the bath at a voltage which is maintained above that at which anode polarization begins and below that at which permanent discoloration or dulling occurs, as set forth above. Preferably suitable agitation of the electrolyte is set up and provided before the electric current is applied and kept constant.

The following is a typical example of how the proper voltage mav be determined, especially with respect to the polishing of silver:

A bath was prepared containing 0.77 mol per liter of potassium cyanide and less than .001 mol per liter of silver. The anode was a plate whose immersed area averaged 2.7 square inches, and which was made from a 999+commercial silver anode.

Thel dimensions of the container for the electrolyte were 10.0" long, 4.75" wide and the liquid 7.5 deep. Two cathodes were used,A having an `aggregate effective area of about 50 square inches, and were equally spaced from the circular path of the anode. at distances of 2%", and the anode was rotated in a circle of 1%" radius approaching to a point approximately .8" from the side Walls of the container at a linear rate of 15.6 feet per minute.

The following is a table showing typical voltage-amperage relationship in a 5% KCN bath:

A vcrafle Average foliage amps/sq. it. Remarks Dulling of ancrir.

Bright 34e in. at c'irze only. Bright in. at erige only. Bright M in. al. edge only.

Bright all over optimum voltage. Bright all over and amnerage. Golden-brown streak at leading edge.

Practically all golden; slight light gray lni at edges.

Light gray dlm M in. from edges.

Light gray lm all over. Dark gray lilm at edges.

Dark gray lilm all over.

the volt-ampere curve. Once this voltage is determined for a particular set oi conditions, reproducible brightness can be obtained. It is important that the voltage is maintained at the level determined to obtain optimum operating conditions.

'I'he determination and maintenance of the critical voltage is essential to the invention, as is also the presence oi' cyanide radicals in the bath.

The above example illustrates the fact that electro-polishing may be accomplished by the addition of only cyanide radicals to the aqueous bath.

A graph, showing typical relationships 'of potential drop, between the anode and cathode, and the corresponding current densities with reference to the polishing of silver in an electrolytic bath, as described on the chart, is given in Fig. 1, in which curves are plotted for conducting the process (I) without agitation of the anode; (1I) with agitation of the anode at 8.25 feet per minute; and (III) with agitation of the anode at 15.6 feet per minute, respectively, the conditions for effective results being as indicated in the legend.

Although polishing was satisfactorily accomplished by the foregoing method, better reproducibility was desired. Gas bubbles were evolved at the cathode which reduced cathodic efficiency. This gas also diffused to the anode causing pin points on the surface. It was found that if a soluble salt of the metal to be polished were introduced into the bath, in a quantity equal to less than that required to form the complex with all the cyanide' radicals present, improved reproducibility occurred and gassing was eliminated. As an example, a silver salt, AgK(CN)z, was used in the presence of 0.3 mol per liter of free potassium cyanide. and the quantity of the silver was varied up to 0.5 mol per liter. Results showed polishing in all concentrations, and very satisfactory results when 0.1 mol per liter yol? silver was used.

The use of the cyanide bath alone requires an optimum voltage which is higher than is economically desirable. A water-soluble electrolyte, such as potassium carbonate, may be used to lower the optimum voltage. The optimum quantity was in the order of 0.3 mol per liter, although any concentration below that at which a black insoluble lm is formed on the anode is effective'. In certain experiments a silver anode was used together with a bath containing in addition to the carbonate of 0.3 mol per liter of free potassium cyanide and 0.1 mol per liter of silver. Sodium sulfate was substituted for the carbonate and equally favorable results obtained with corresponding concentration.

To increase the reproducibility of results,` the anode is preferably agitated in any bath, in accordance with the present invention, so that there is a uniform ow of solution over all parts of the anode. For example, violent motion interferes with the control of voltage', `but agitation at a uniform speed of about 20 to 40 inches per minute is satisfactory, using the bath mentioned in the paragraph above. The best voltage was 1.58 Volts at 20 inches per minute, and 1.75 volts at 40 inches per minute, thus as the rate of agitation increases, the optimum voltage increases. The materials to be polished should be hung in the most streamlined position possible to avoid turbulence. The following table gives optimum current densities and voltages at different circular agitation rates for teaspoons in the bath mentioned above. The rates are vapproximate values only.

-It is seen that they vary nearly linearly with eachother.

Rate, inches per minute Besggolt v rtagt It was observed that in the foregoing experi-` ments satisfactory polishing was obtained if the plate to be treated was still wet after plating.. In fact it is generally most convenient to carryv onj the polishing operation in conjunction with and directly after a plating operation. In any case it is highly desirable that the plate be clean.

An example of optimum silver bath is as follows:

. Mol

AgK(CN)z per liter solution 0.1 l

j KCN (free) per liter solution 0.3

Na2SO4 per liter solution 0.2

Agitation at the rate of 50 inches per minute and anide per liter solution, at 5.0 volts, 300 amps. per square foot, and agitation at 40 feet per minute, a. bright polish is obtained. If 0.1 mol of` cadmium cyanide Cd(CN)z per liter is added to feet per minute, a brightpolish is obtained at v1.5 volts and 80 amperes per square foot.

An example of copper polishing is as follows:

' With a bath containing 2 mols potassium cyanide per literof solution at 3.0 voltsvandrllO amps. per square foot, and with agitation at 40 feet per minute, a bright polish is obtained. f

The bath measured the same` this bath and agitation is uniform at about 8` Brass was polished in a bath containing 1 mol of potassium cyanide per liter at 8 volts and 86 amps. per square foot and with agitation at 20 feet per minute.

It is to be understood that the scope of this invention is not limited to the particular condii l tions described in the examples but includes the range of variation from such conditions lasI indicated for silver. i

causes silver'from the anode to enter into Athe surface illm of electrolyte adjacent to the anode, thus balancing the loss of silver froml such illm by thermal diifusion or agitation into the solution. If the voltage is too low, the diffusion` rate exceeds the rate of formation and the lm disintegrates, producing a dull anode. If the voltage rises considerably'above the amount necessary to maintain this film, gas which results from decomposition of the solution at the high voltages forms at the anode and breaks the lm momentarily. This is visually apparent. The constant removal of silver from high spots in excess of that from the low spots soon erases all surface roughness and the anode takes on a polished appearance.

In the practical application of the invention, it is observed that under the conditions above described a good polishing effect requires suiiicient time for the complete removal of the surface protuberances which interfere with the reflection of light so that the whole may be reduced to a geometrically perfect light-reflecting surface which is characteristic of polished surfaces. f On the other hand, the time required to produce this polished effect will vary with the irregularity or roughness of the original surface to be polished. f

If extremely rough, obviously too much metal Would require removal for economical employment of the metal. In such cases frostiness may persist in such areas or surfaces, unless the action is sufficiently prolonged. l

It is further observed that good polishing may be accomplished even though the original surface be grainy and of poor uniformity.w Milkiness also may be present, without preventing `a satisfactory polish.

When cyanide is used alone, blisters may occur but are not serious even then.

Low reflectivity or polish may occur if too short a time is allowed for the treatment, but, as a1- ready pointed out, this is to be expected and will4 cadmium, valloys consisting of these metals, brass/ and nickel silver', in cyanide baths, the improvetrodefs,whichisl below the point at which periodic a minor proportion of nickel and/or zinc as com-I pared with one or more of the metals falling in the group of silver, copper and cadmium will also polish by the process of this invention. Alloys of 25% nickel and 75% copper and .nickel silver were polished by this method. `Nickel /an'd zinc .f Athemselvesdo not, however, polish satisfactorily.

If the concentrationof the silver or other metal is increased and the concentration of free cyanide the electric potential at the anode surface is maintained at the proper level, thpotential ',i'la'shingor permanent film" formation occurs, and It is also to be noted that alloys which contain, 1,

as Aan aid in'A establishing and maintaining the 'constant `potential difference,` moving the anode with respect lto the `electrolyte and solid objects in-ssuch a mannerl that turbulence is avoided and thev anode does not approach another solid so closely that flashing is-produced, and maintaining these. conditions lfor a period of time to effect the dissolution of superficial protuberances from the surface and vto leave the surface polished.

2. In the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass andv'nickel silver, vin cyanide'baths wherein the anode is moved in a vcontinuous unidirectional path with respect to the electrolyte and solid objects, and wherein the potential difference is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanentV film formation occurs, the method of inhibiting flashing which comprises maintaining the interelectrode potential difference substantially constant, and main-g taining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface and to leavel the surface polished.

3. In the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass and nickel silver, incyanide baths wherein the anode is moved in a continuous unidirectional path with respect to the electrolyte and solid objects, and wherein the potential difference is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanent lm formation occurs, the method of inhibiting flashing which comprises controlling the movement of the anode so as to preclude turbulence, and maintaining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface and to leave the surface polished.

4. In the anodic polishing of silver, copper, cadmium, alloys consisting of these'metals, brass and nickel silver, in cyanide baths wherein the anode is moved in a continuous unidirectional path with respect to the electrolyte and solid objects, and wherein the potential difference is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanent filmy formation occurs, the method of inhibiting flashing which comprises controlling the movement of the anode so that the anode does not approach any other solid so closely that flashing is "produced, and maintaining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface -and to leave the surface polished.

5. In the anodic polishing of silver in cyanide baths wherein the anode is moved in a continuous undirectional path with respect to the electrolyte and solid objects, and wherein the potential diffe'rence is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanent film formation occurs, the method of inhibiting flashing which. comprises maintaining the interelectrode potential difference substantially constant, and maintaining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface and,

to leave the surface polished.

6. In the anodic polishing of copper in cyanide baths wherein the anode is moved in a continuous unidirectional path with respect to the electrolyte and solid objects. and wherein the potential difference is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanent film formation occurs, the method of inhibiting flashing which comprises maintaining the interelectrode potential difference substantially constant,k and maintaining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface and to leave the, surface polished.

7. In the anodic polishing of brass in cyanide baths wherein the'anode is moved in a continuous unidirectional path with respect to the electrolyte and solid objects, and wherein the potential difference is maintained above the point at which polishing begins to take place and below the point at which periodic flashing or permanent film formationoccurs, the method of inhibiting flashing which comprises maintaining the interelectrode potential difference substantially constant, and maintaining the conditions for a period of time to effect the dissolution of superficial protuberances from the surface and to leave the surface polished.

SAMUEL E. EATON.

REFERENCES CITED The following references are of record in the le of this patent:

FOREIGN PATENTS Country Date German Sept 24, 1910 OTHER REFERENCES Number Gilbertson et al., The Electrochemical Soc.

Uhlig, article and discussion in Transactions of the Electrochemical Soc., vol. '78 (1940), pages 265-273. (Copy in Division 56) Raub et al., article in Mitt. Forsch-Inst. Edelmetalle staatl. hoheren Fachschule schwab. Gmund., vol 6, pages 1-14 (1941), abstracted in Chemical Abstracts, vol. 37, page 2997. 

